The present invention is directed to a method for improving the deposition of patterned thin films using a scanning localized evaporation methodology (SLEM) incorporating a collimating mask assembly for producing multilayered electronic and photonic devices, such as transistors, sublimable organic light-emitting diodes (OLEDs), photonic band gap structures, and integrated circuits and systems.
The growth of ultra-thin organic films currently involves predominantly the use of high vacuum,[1, 2] Langmuir-Blodgett film deposition,[3–5] or self-assembled monolayers.[6–11] Typically, producing high resolution patterns in these films requires the use of lithographic techniques that enable the selective removal of portions of the deposited layers.[12, 13] Lately, soft stamping[14–18] and ink-jet printing[19–22] have permitted direct patterned depositions of polymeric semiconductors. More recently, a scanning localized evaporation methodology (SLEM) has been invented.[23] This process features close proximity, selective deposition of thin films in patterns suitable for fabrication of electronic, optoelectronic and photonic devices. In addition, high deposition rates and improved material economy are realized.
As a result, the definition of high fidelity patterns in deposited thin films is generally obtained through the use of a variety of photolithographic and etching techniques. Ink-jet printing technologies usually employ microwells to contain deposited microdroplets of ink and limit its spreading while drying. Direct vacuum deposition of patterned films requires the use of shadow masks that usually are in direct contact with the substrate, in which case they are termed contact masks. Contact masks typically exhibit the following limitations:                i) fine-sized features are prone to clogging due to material deposition along the edges of the openings;        ii) in the contact mode, mask removal can cause scratching of the deposited pattern;        iii) large area contact masks are prone to warping and distortion;        iv) circular stand-alone pattern elements cannot be supported (such as the letter “P”, “O”, etc . . . ).        
In the case of scanning localized evaporation methodology (SLEM), the above limitations are partially addressed through:    1. heating of the mask to avoid material deposition along the edges of the openings and to prevent the accompanying clogging of fine features;    2. separating the mask from the substrate by a finite distance, thus preventing scratching of the deposited pattern;    3. the use of the scanning feature in SLEM to permits use of small area shadow masks, thereby considerably reducing the warping and distortion,    4. Circular stand-alone pattern element can be implemented by either of two methods:            i) The scanning of a smaller spot to define the desired pattern                    ii) Superposition of one or more patterns to construct annular elements (i.e. in the case of “P”, use of “I” and “⊃”).                        
As discussed above, pattern definition in the SLEM technique is greatly dependent on the mask to substrate separation. Reduction in this spacing between the mask and substrate improves the resolution. However, increased radiation adversely affects the quality of the deposited films as the spacing between substrate and heated mask decreases.
FIG. 1 schematically illustrates the basic operating principle of the SLEM process as developed by Applicants and which is the subject matter of our prior application for Letters Patent Ser. No. 10/159,670, filed Jun. 3, 2002. Herein, on a cylindrical transport mechanism 1, an array of heating elements 2 is mounted, each of which can be energized individually through appropriately placed electrodes 3 selectively powered with electrical commutators 4 at desired locations (i.e., opposite a shadow mask 7). A loading source 5 deposits the evaporant 6 on the heating elements 2. Upon rotation to a position adjacent the substrate 8, the evaporant material 6 is re-evaporated from the surface of the selected heating element 2 and passes through the shadow mask 7. The mask generally consists of a rigid plate with openings 100 forming a pattern dictated by the structural requirements of the device under fabrication.
FIG. 2 illustrates a magnified cross-section around the shadow mask. This includes a portion of the rotor 1 with a number of heating elements 2 between a set of electrodes 3. The heating elements are powered by two electrical commutators 4. The width of the commutators 4 and the angular speed of the rotor 1 defines the time of evaporation. During this time the heating element 2 between the two electrodes containing the commutators is powered to re-evaporate the evaporant 6, which has been earlier deposited at the loading station 5 (shown in FIG. 1). Resistive heating of the shadow mask 7 through the electrical terminals 9 prevents the deposition of the evaporant onto the mask, thereby keeping the fine mask openings 100 free from clogging.
Because of the natural divergence of the evaporant flux, obtaining high-fidelity patterns requires close proximity of the mask to the substrate shown in FIG. 2A as opposed to the configuration of FIG. 2B, which has a larger mask to substrate separation. The configuration of FIG. 2A however, presents a number of limitations: (a) close proximity is prone to mechanical damage of the evaporant deposit film when the substrate is translated to another location; and (b) fine patterns generally become clogged due to material deposition along the edges of the openings, resulting in the gradual distortion of pattern shape and sizes. Mask clogging can be avoided by heating the mask to a temperature at which material deposition on the mask does not take place. The heating of the mask in close proximity to the substrate can however adversely affect the quality of the deposited layer.
It is an object of the present invention to provide a novel SLEM method using collimating apparatus for generation of closely controlled patterns on the substrate.
It is also an object to provide a novel collimating mask assembly for generating closely controlled patterns on a substrate.